What type of sentence is "The teacher returned the homework after she noticed the error"?

The sentence "The teacher returned the homework after she noticed the error" is classified as a complex sentence because it contains an independent clause and at least one dependent (or subordinate) clause. In this case, "The teacher returned the homework" is the independent clause, which can stand alone as a complete thought. The dependent clause "after she noticed the error" cannot stand alone and provides additional information about when the action of the independent clause took place. This combination of an independent clause and a dependent clause is the defining characteristic of a complex sentence.

In contrast, a simple sentence consists of only one independent clause, a compound sentence consists of two or more independent clauses joined by a coordinating conjunction, and an interrogative sentence asks a question. These other types do not apply here because the sentence structure features a dependent clause, highlighting its complexity.